Wear-resistant nickel-aluminum coatings



United States Patent Ofiice 3,141,744 Patented July 21, 1964 3,141,744WEAR RESISTANT NlCKEL-ALUM CGATHIGS Dwight E. Couch, Boulder City, Nam,and Harold Shapiro, Silver Spring, and Abner Brenner, Chevy Chase, Md,assignors to the United States of America as represented by the ecretaryof the Navy No Drawing. Division of application Ser. No. 829,156, .luly23, 1959, now Patent No. 3,046,205, dated .luly 24, 1962. Continuationof application Ser. No. 665,616, June 13, 1957. This application June19, 1961, Ser. No. 11%,197

Ciaims. (Cl. 29-194) (Granted under Title 35, US. Code (1952), see. 266)The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This application is a continuation of abandoned application Serial No.665,616, filed June 13, 1957, for Nickel- Aluminum Alloy Coatings and isa division of application Serial No. 829,156, filed July 23, 1959, nowPatent No. 3,046,205, granted July 24, 1962, for Nickel-Aluminum AlloyCoatings.

The present invention relates generally to hard coatings for wear andoxidation resistance. Such coatings can be produced on metallic surfacesfor increasing wear resistance of sliding or reciprocating parts ofmachines and the like, such as bearing surfaces, pistons, piston rings,cylinder walls and gun mechanisms. More particularly, the inventionpertains to composite articles of manufacture comprising a base metalwith a coating of nickel-aluminum alloys and methods of applying thesecoatings so that a layer of unalloyed nickel remains between thenickel-aluminum layer and the base metal.

In the coating art, it has been the general practice to employ metalswhich have characteristically high wear, oxidation and temperatureresistance for the surface layers. For example, coatings of chromium andchromium alloys on metal surfaces have been used where surfaces aresubjected to sliding friction. Chromium plated piston rings are atypical product of this approach.

However, these surface layers of pure metals or alloys thereof withother metals to form intermediate solid solutions exhibit typicalmetallic characteristics. Although these characteristics vary in degreefrom metal to metal they all have properties of hardness, ductility andtoughness in a certain order of magnitude since the valence binding inthese metals is the metallic bond characteristic of all metals and solidsolution alloys. To obtain a surface layer of a new order of magnitudesimple metals or intermediate solid solution alloys will not suifice.

It is, therefore, an object of the present invention to provide basemetals such as steel, nickel or copper with coatings of nickel-aluminumwhich exhibit wear, oxidation and temperature resistance far beyond thatwhich could be obtained with coatings of either nickel, aluminum, ornickel-aluminum intermediate solid solution alloys. This objective iselfected by producing a coating with proper stoichiometric proportionssuch that the nickel and aluminum form intermetallic compounds whichpossess physical properties highly suitable for wear, oxidation andtemperature resistance. In the nickel and aluminum system theseintermetallic compounds consist of nickel aluminides such as Ni Al whichcompounds exhibit an intermediate type of binding between covalent andmetallic.

It is a still further object of the present invention to provide anickel-aluminum coating which is free of unalloyed aluminum. Thepresence of such aluminum will considerably detract from the desiredwear resistance characteristics since, as is well known, aluminum isextremely ductile and would tend to gum the sliding surfaces.

Another objective of the present invention is to produce an article witha layer of unalloyed nickel between the base metal and the surface layerof nickel aluminide in order to afford a better bond between the basemetal and the nickel aluminide coating thus assuring a firm, tenaciouscoating.

Still another object is to produce an article with a carefullycontrolled and readily reproducible deposit of aluminum on a nickelsurface thereby permitting diffusion of the aluminum into the nickel tothe point where no free aluminum remains but free nickel does remainbetween the thus formed nickel aluminide and the base metal.

Yet another object of the present invention is to provide a coating ofnickel aluminide of a carefully controlled thickness such that theinherent brittle characteristics which intermetallic compounds exhibitin massive form will not have occasion to manifest themselves.

It is still another object to provide a coating of wearresistant nickelaluminide which will retain its room-temperature hardness after heatingto 1000 C. and cooling and will be more oxidation and temperatureresistant than either nickel, aluminum, or intermediate solid solutionsthereof.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description.

Nickel-aluminum intermetallic compounds possess many desirableproperties over the separate forming metals, among the chief of whichare a substantial increase in melting point, stability at hightemperatures, good modulus-of-rupture strength, excellent oxidationresistance and hardness. With these desirable properties, however,nickel aluminide, in massive form, is difficult to fabricate and toobrittle for practical applications. To overcome these difiiculties forcoating use, the nickel aluminide is formed directly on the base metalsurface after the necessary shaping operations have been completed. Thisis accomplished by electrodeposition process steps and several modes ofaccomplishing this result will now be described.

In the first method, nickel is deposited on the base metal in theconventional Way, as by electro-deposition from baths containing nickelsulfate or nickel chloride. If the nickel when deposited is severelysoiled, degreasing of the article with subsequent pumice scrubbingthereof is required prior to aluminum plating. Unless the nickel ismarkedly soiled no precleaning is necessary. Aluminum is then depositedon the nickel plate by electrodeposition from a fused salt bathconsisting of an aluminum halide and an alkali halide, as, for example,aluminum chloride and sodium chloride in the ratio of upwards of 1.5moles of aluminum chloride to 1 mole of sodium chloride. Satisfactoryperformance results from operation at 1 to 4 amperes/ sq. drn. and at atemperature in the range of -200 C. Once prepared the bath must be keptmolten, for if allowed to solidify it is very difficult to remeltwithout breaking the vessel containing the bath. The time of treatmentvaries depending on the thickness of plating desired but it mostcommonly varies between 5 minutes and 1 hour. Very good, coherentaluminum deposits up to 0.8 mil thick'can be produced by this methodusing aluminum (2S) anodes. When thicker deposits are desired smoothdeposits of aluminum up to 0.002 inch thick can be obtained by usingtungsten anodes.

After the aluminum has been deposited on the nickel coat the article isdried and placed in a furnace preferably of the electric type such as ahigh frequency induction or resistance furnace. Heating may be conductedin air or in an inert atmosphere, such as in helium or nitrogen, at atemperature in the range from 550 to 750 C. for periods up to abouthours. At this temperature all the aluminum alloys with the nickel toform a nickel aluminum alloy layer over the unalloyed portion of thenickel layer. This arrangement is accomplished by limiting the depositof aluminum to that of a layer having a thickness less than thethickness of the nickel layer. Actually, the optimum deposit thicknessesare an aluminum deposit approximately one-fourth the thickness of thenickel layer whereby all the aluminum is alloyed with the nickel forminga layer of nickel aluminides exceeding in thickness the prior aluminumdeposit but still leaving a portion (about one-half) of the originalnickel layer in unalloyed condition.

Actually, it has been found that the oxidizing effect of heating in airis beneficial in that there is a marked increase in resistance to saltspray corrosion. In case the H alloy is formed by heatin in an inertatmosphere the oxidation will in any event occur during normalsubsequent high temperature applications.

The first method, as above described, in particularly useful where thethickness of the aluminum deposit is less than 0.001 inch or where theobject being coated might be damaged by heating to too high atemperature. However, as stated above thicker deposits can be obtainedby the use of tungsten anodes.

In the second method aluminum is electrically deposited on the nickelcoat from a fused salt bath operated at a temperature of about 660 to1000 C., i.e., above the melting point of aluminum, the alloy beingformed simultaneously with the plating operation due to the heat of thebath. A suitable bath is cryolite at a temperatule of 1000 C. orcryolite dissolved in alkali halides such as sodium chloride andpotassium chloride at a temperature ranging from 660 to 900 C. Usingcryolite with the bath temperature at about 1000 C., and a current flowof 50 amperes per square decimeter surface, a flow of 2 to 3 minuteswill produce a coat of about 0.0005 inch thickness. A coat thickness of0.001 to 0.002 inch is produced by a current of 20 to amperes per squaredecimeter flowing from 15 to minutes. For deposits over 0.002 inch 15 to20 amperes per square decimeter flowing for about 1 hour are required.

In the operation of the potassium chloride, sodium chloride, cryolitebath although the bath may be operated in the temperature range stated(660 9 00 C.), at the higher temperature the graphite anodes oxidizerather rapidly if an inert atmosphere is not used While at the lowertemperature the aluminum diffuses slowly into the nickel. A series ofruns has shown that a temperature range of about 700 to 800 C. gives asatisfactory compromise of these factors. When the bath is used in thistemperature range, aluminum deposits 0.25 mil thick are produced in 5minutes at a current density of 7 amperes/sq. dm. To

produce thicker deposits of aluminum at a rate not in excess of the rateof diffusion of the aluminum into the nickel the following scheduleshould be employed in using this bath in the optimum temperature range.

Since the bath used to electrodeposit the aluminum is operated attemperatures higher than the tempering temperature of steel, when thebase material is steel or a steel alloy, heat treatment becomesnecessary to restore the temper of this base material after the nickelaluminide layer has been formed.

Samples to be plated were placed in the bath and allowed to hang therefor 10 to 20 seconds to reach bath temperature before the current wasturned on. Optimum concentrations for the chloride-cryolite bath are:Sodium chloride 440 gms., potassium chloride 560 gms. and cryolite 150gms. Optimum operating conditions exist at a current density of 2 to 10amps/ sq. dm. and in the temperature range of 700 800 C.

In order to avoid producing excess unalloyed aluminum on the surface itis often advantageous to employ varying current densities. Thus, in caseof a chloride-cryolite bath operated at 700 C. deposits of 1 mil thickcan be prepared by plating at 7 amp/sq. dm. for 5 minutes and thenreducing the current density to 3-4 amp/ sq. dm. and plating for 30 to40 minutes. Deposits of 1.8 mils can be produced in two hours by thefollowing schedule: 7 amp/sq. dm., 5 minutes; 4 amp/sq. dm., 20 minutes;2 amp/sq. dm., minutes. These plating rates can be increased if the bathtemperature is raised above 700 C.

An excessive rate of deposition of aluminum is to be avoided since atbath temperature this aluminum is molten and nickel dissolves in moltenaluminum. The problem created is that of depletion of nickel from thenickel layer since the nickel goes into solution in the molten aluminumand the aluminum collects into beads which subsequently fiow into thebath carrying the dis solved nickel along. This results in a reductionin the amount of nickel available for alloying.

This is in fact one of the basic disadvantages of the hot-dip processknown in the prior art. As an indication, some 4" x 6 samples wereplated with about 0.001 inch of nickel and accurately weighed. Thesesamples were then sent to a commercial firm for the deposition thereonof aluminum by the hot-dip process. All samples actually lost weight asa result of the dipping process as indicated by the following:

Wt. of nickel sample Wt. of Al sample base coated after deposit strippedof (gins) base hot-dip (gins) unalloyed (gms) (gms) Al deposit Thenickel aluminide, as produced by the described methods, has a hardnessvarying from 700 to 0 Vickers at room temperature depending on thepercentage of aluminum in the given nickel aluminide. This fact makesthis compound particularly useful for wear resistance in pistons, pistonrings, cylinder walls, gun mechanism, sliding parts of machines and thelike. In addition, the room temperature hardness of the compound is notaffected by heating to 1000 C. whereas, in the case of chromium thehardness is destroyed.

Further, the nickel aluminide is markedly oxidation resistant even at1100 C. there being only slight surface oxidation. This oxidationresistance is apparent when comparison is made with nickel in which,over a 400 hour period, there is a gain in weight at 1100 C. of 0.05gram per cm. as compared with an 0.01 gram gain for nickel aluminide atthe same temperature and for the same time period. Since nickel andchromium have approximately the same oxidation properties, it isapparent that the oxidation resistance of nickel aluminide exceeds thatof chromium, also. Since the described methods all involveelectro-deposition, the coating thickness may be readily controlled,values ranging from 0.0002 to over 0.002 inch being readily obtainable.

In addition to the beneficial results already set forth the results ofsalt spray corrosion tests (set forth below) on a series of steel alloypanels coated with nickel aluminum alloy clearly indicate the excellentprotection afforded in this respect by the present invention. Further,these tests indicate the superior nature of coatings in which a layer ofnickel remains between the base and the nickel aluminide layer.

Percentage of Thickness Thickness surface covered of Ni in of Al in withrust after inches inches a minimum of 75 hours) 1 salt spray 0.0010.0003 1-2 0.0002 0.001 over 50 0.0005 0.0003 3-5 0. 002 0. 0003 lessthan 1 0.0005 None 80 0. 0004 0.00026 5 1 0. 0004 0.00026 None 1Oxidized.

While any of the structural metals may be used as the base material, incommon use for this purpose are steel, nickel or copper.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A composite metal article containing a hard coating which increaseswear resistance of sliding or reciprocating parts comprising incombination a base metal, a layer consisting of unalloyed nickel bondedto and covering said base metal and a surface layer consisting of nickelaluminide bonded to and covering said nickel layer, said surface layerbeing free of contamination from the base metal and free of unalloyedaluminum.

2. The composite metal article of claim 1 wherein the base material is ametal having a melting point higher than aluminum.

3. The composite metal article of claim 2 wherein the layer of unalloyednickel is approximately of the same thickness as the surface layer ofnickel aluminide.

4. The composite metal article of claim 3 in which the nickel aluminidecoating has a room temperature hardness between 700 and 1050 Vickers andretains this room temperature hardness after heating to 1000 C. andcooling.

5. The composite metal article of claim 1 wherein the aluminum contentis about one-fourth the nickel content.

References Cited in the file of this patent UNITED STATES PATENTS2,123,686 Spencer July 12, 1938 2,490,548 Schultz Dec. 6, 1949 2,682,101Whitfield et a1. June 29, 1954 2,709,154 Hansgirg May 24, 1955 2,752,667Schaefer July 3, 1956 2,837,818 Storchheim June 10, 1958

1. A COMPOSITE METAL ARTICLE CONTAINING A HARD COATING WHICH INCREASESWEAR RESISTANCE OF SLIDING OR RECIPROCATING PARTS COMPRISING INCOMBINATION A BASE METAL, A LAYER CONSISTING OF UNALLOYED NICKEL BONDEDTO AND COVERING SAID BASE METAL AND A SURFACE LAYER CONSISTING OF NICKELALUMINIDE BONDED TO AND COVERING SAID NICKEL LAYER, SAID SURFACE BEINGFREE OF CONTAMINATION FROM THE BASE METAL AND FREE OF UNALLOYEDALUMINUM.